1 roll heavy duty aluminum foil.
scrap foam or Styrofoam
Step 1: Position and tape down your foam on to the heatpump.
most old heat-pumps are in the shape of a box with a round fan molded to the end of it. We are going to insulate the box part.
tape your foam to the surfaces that feel cool to the touch, or if you have had the panels off, put your foam on the surfaces that cover the air handler.
I deconstructed an old foam drink cooler for my insulation panels.anything from 1-3" thick will work.
Step 2: Cover the foam with aluminum foil
when I did this little modification, my house temp went down by 2-3 degrees.
Optionally, you could also paint the fan shroud and any other exposed metal with white spray paint.